1. Field of the Invention
The present invention generally relates to the relief of dielectric stress at high voltage terminations and is particularly directed to the type of high voltage cable termination wherein a shielded high voltage cable is terminated with the shielding of the cable conductively coupled to the ground electrode.
The term "termination" as used herein includes not only those applications wherein a cable is terminated at a source or at a load but also those applications wherein a cable is spliced to another cable. The term "ground electrode" as used herein means an electrode to which the cable shielding is conductively coupled, whether or not such electrode is adapted for contact with an earth ground.
2. Discussion of the Prior Art
A number of prior art means for providing relief of dielectric stress at high voltage cable terminations have been proposed in U.S. Pat. No. 3,349,164 to Wyatt; U.S. Pat. No. 2,745,897 to Nicholas; Great Britain patent specification No. 1,129,887; U.S. Pat. No. 3,585,274 to Tomaszewski and Wyatt; and in the patents therein noted. These prior art means generally have been directed to controlling dielectric stress by more evenly distributing the gradient of the electric field in the insulation of the high voltage cable at the location of the ground electrode.
The means described in U.S. Pat. No. 3,585,274 are very effective in more evenly distributing the electric field gradient. According to the teaching therein, stress relief is provided by a tube of a high permittivity (specific inductive capacity) material having a relatively smooth geometry. The tube is positioned in a housing for surrounding an insulated high voltage cable with one end of the tube in coaxial contact with a ground electrode, which electrode is itself positioned for conductive coupling to the shielding of the high voltage cable. For the embodiment described therein (see FIG. 4 of the patent), wherein only the ground electrode is coaxially contacted by the tube, it is stated that the permittivity of the tube is about at least five to 10 times the permittivity of the insulation of the high voltage cable. This minimum is dependent upon the thickness of the tube and the voltage rating of the device to which the high voltage cable is to be connected. Such an embodiment rated at about 35 Kilovolts or less is said to operate with a tube permittivity of about 15, although a tube permittivity of at least 30 is preferred.
In another embodiment also described in U.S. Pat. No. 3,585,274, (see FIG. 3 of the patent) the tube makes coaxial contact with both the ground electrode and a high voltage electrode which is positioned for contacting the conductor of the high voltage cable. For this embodiment, which is said to be preferable for applications rated at 15 kilovolts or higher, it is stated that the permittivity of the tube is about at least 50 to 100 times the permittivity of the insulation of the high voltage cable, and that while this embodiment is operable with a tube permittivity of about 50, a tube permittivity of at least 200 is preferred.
The tube used in the embodiments described in U.S. Pat. No. 3,585,274 is a ceramic material including one or more titanates and/or titanium dioxide. Although such a tube is considerably more expensive than the elastomeric materials commonly used for providing dielectric stress relief in other prior art devices, it does provide the high permittivity of at least 50 that is desired for the latter above described embodiment, which high permittivity has not been attainable with elastomeric materials.